1. Field of the Invention
This invention generally relates to a hybrid radio communication system which may be switched between verbal communication and data communication, and, more particularly, to a dc decoupled hybrid radio communication system in which signals exchanged between a radio unit and an interface circuit are dc decoupled. More specifically, the present invention relates to a hybrid radio communication system which may selectively serve as an ordinary radio set for verbal communication or as a radio facsimile for data communication.
2. Description of the Background
A hybrid radio communication system which may serve as a radio set for verbal communication and a radio facsimile machine for facsimile communication is well known and disclosed, for example, in Japanese Pat. Laid-open Pub. No. 61-13764 and Japanese Utility Model Laid-open Pub. No. 60-72048. In general, when carrying out radio facsimile communication using a radio unit, an interface circuit 4 is typically provided between a radio unit 1 and a facsimile unit 2 as shown in FIG. 3. The interface circuit 4 serves to provide impedance matching between the radio unit 1 and the facsimile unit 2 by adjusting the level of each of the signals exchanged between units 1 and 2. It also serves to establish an operative coupling between the radio unit 1 and a microphone 3 during the verbal communication mode or between the radio unit 1 and the facsimile unit 2 during the facsimile (data) communication mode.
If such a hybrid radio communication system is mounted on a vehicle, such as an automobile, then power is supplied by a battery 5 to each of its components as also shown in FIG. 3. Thus, the radio communication system shown in FIG. 3 defines a mobile radio communication system. When this system is in operation, power supply current flows along the paths indicated by the solid lines in FIG. 3. In general, the signal lines 6 between the radio unit 1 and the interface circuit 4 and also the signal lines 7 between the interface circuit 4 and the facsimile unit 2 have ground lines which are dc coupled to each other. For this reason, part of the power supply current tends to flow along these ground lines of signal lines 6 and 7 as diagrammatically indicated by dotted lines d and e in FIG. 3. With part of the power supply current flowing along the ground lines, a resulting voltage drop is superimposed on the signal which is being transmitted, thereby creating a noise problem. The interface circuit 4 is typically disposed close to or integrally with the facsimile unit 2 and thus the signal lines 7 extending therebetween are rather short, so that their line resistances and voltage drops are small. Accordingly, no practical problem is normally presented in signal lines 7.
On the other hand, the radio unit 1 is normally separated from the interface circuit 4, and, thus, the signal lines 6 between the radio unit 1 and the interface circuit 4 tend to be long and their line resistances tend to be large, thereby providing relatively large voltage drops. Therefore, a noise problem could arise in the signal lines 6, which could deteriorate the quality of transmitted data and of the resulting image. Besides, in the prior art system shown in FIG. 3, noises produced at the radio unit 1 or at the facsimile unit 2 could enter into the other unit, which could deteriorate the quality of verbal and data communication.
In order to prevent noises from being transferred from one component to another in such a radio communication system, a transformer may be used to decouple dc components between the component circuits as taught in Japanese Pat. Laid-open Pub. No. 62-125720. Japanse Utility Model Laid-open Pub. No. 60-82869 teaches a hybrid radio communication system for verbal and facsimile communications using a transformer. However, this publication only teaches the use of a transformer for impedance matching.